Light emitting element and method of making same

ABSTRACT

A light emitting element has a substrate of gallium oxides and a pn-junction formed on the substrate. The substrate is of gallium oxides represented by: (Al X In Y Ga (1-X-Y) ) 2 O 3  where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. The pn-junction has first conductivity type substrate, and GaN system compound semiconductor thin film of second conductivity type opposite to the first conductivity type.

STATEMENT OF RELATED APPLICATIONS

This application is based on Japanese Patent Application Nos.2003-137912 and 2002-160630, the entire contents of which areincorporated herein by reference. Also, the present application is acontinuation of currently pending U.S. patent application Ser. No.11/982,580 filed on Nov. 2, 2007, which is a divisional of U.S. patentapplication Ser. No. 11/211,860, filed on Aug. 25, 2005, now U.S. Pat.No. 7,319,249 B2, which is a continuation of U.S. application Ser. No.10/452,158, filed May 30, 2003, now U.S. Pat. No. 6,977,397, issued Dec.20, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light emitting element with a wide bandgapenough to emit visible light to ultraviolet light and a method of makingthe same, and relates particularly to a light emitting element employinga colorless, transparent and conductive substrate that transmits emitvisible light to ultraviolet light, offering a vertical structure inelectrode configuration, and allowing emitted light to be outputted fromthe substrate side and a method of making the same.

2. Description of the Related Art

Conventionally, a light emitting element with a composition of SiCsubstrate/n-GaN/p-GaN is known (e.g., Japanese patent applicationlaid-open No. 2002-255692).

SiC is brown and transparent material, and it transmits visible light upto about 427 nm. Therefore, a light emitting element employing the SiCsubstrate allows emitted light to be outputted from the substrate side.

The light emitting element employing a SiC substrate is manufactured byepitaxially growing SiC thin film on a SiC single crystal wafer to getthe SiC substrate, then growing n-GaN and p-GaN layers on the substrate,cutting out light emitting element chips.

However, there is a serious problem in the light emitting elementemploying the SiC substrate. The SiC single crystal wafer has a badcrystalline quality such that there exist micro-pipe defects penetratingvertically in the SiC single crystal wafer. Therefore, it is requiredthat, when making chips from a wafer having n-GaN and p-GaN layers grownthereon, the wafer must be carefully cut while avoiding the micro-pipedefect. This causes a bad efficiency in the manufacture of lightemitting element.

On the other hand, SiC transmits up to light of blue region but does nottransmit light of ultraviolet region. When GaN-emitted light includingvisible light to ultraviolet light is outputted from the substrate side,the light of ultraviolet region cannot be transmitted therethrough.Thus, ultraviolet light cannot be outputted from the substrate side.Furthermore, SiC is brown-colored and, therefore, when transmittinglight through SiC, part of emitted light wavelength must be absorbed.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light emitting elementthat employs a colorless, transparent and conductive substrate thattransmits emit visible light to ultraviolet light, offers a verticalstructure in electrode configuration, and allows emitted light to beoutputted from the substrate side and a method of making the same.

It is another object of the invention to provide a light emittingelement with a good manufacturing efficiency and a method of making thesame.

According to the invention, a light emitting element comprises:

a substrate of gallium oxides; and

a pn-junction formed on the substrate.

According to another aspect of the invention, a light emitting elementcomprises:

a single crystal substrate of oxides including gallium as the majorcomponent; and

compound semiconductor thin film formed on the single crystal substrate.

According to a further aspect of the invention, a method of making alight emitting element, comprises the steps of:

growing a single crystal substrate including gallium as the majorcomponent by EFG(Edge-defined film Fed Growth) method where, in ahigh-temperature vessel of a controlled atmosphere, using a slit diethat allows source material melt to be continually lifted above the slitdie through the capillary phenomenon of a slit provided with the slitdie and a crucible that accommodates the slit die and the sourcematerial melt, single crystal the cross section of which has the sameshape as the top surface of the slit die is grown; and

growing compound semiconductor thin film on the substrate.

According to a further aspect of the invention, a method of making alight emitting element, comprises the steps of:

providing single crystalline Ga₂O₃ system seed crystal and non-singlecrystalline Ga₂O₃ system material;

growing a single crystal substrate including gallium as the majorcomponent by FZ(Floating Zone) method where the Ga₂O₃ system seedcrystal and Ga₂O₃ system material are contacted and heated such that theGa₂O₃ system seed crystal and Ga₂O₃ system material are melted at thecontacting portion, thereby crystallize the Ga₂O₃ system material; and

growing compound semiconductor thin film on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be explained with referenceto the drawings, wherein:

FIG. 1 is a graph showing a temperature dependency of resistivity of.—Ga₂O₃;

FIG. 2 is a partly cross sectional and perspective view showing acrucible 6 to be inserted into FEG pulling vessel used in a method ofmaking a light emitting element according to the invention;

FIG. 3 is a partly cross sectional view showing an infrared heatingsingle-crystal growing apparatus used in a method of making a lightemitting element according to the invention;

FIG. 4 shows an atom arrangement in the case that GaN (001) face thinfilm is grown on (101) face of .—Ga₂O₃ system single crystal substrate;

FIG. 5 shows an atom arrangement in the case that GaN (001) face thinfilm is grown on (001) face of Al₂O₃ system crystal substrate;

FIG. 6 is an illustration showing an MOCVD apparatus used in a method ofmaking a light emitting element according to the invention;

FIG. 7 is a cross sectional view showing a first example of lightemitting element according to the invention;

FIG. 8 is a cross sectional view showing a modification of the firstexample in FIG. 7;

FIG. 9 is a cross sectional view showing a second example of lightemitting element according to the invention;

FIG. 10 is a cross sectional view showing a third example of lightemitting element according to the invention; and

FIG. 11 is a cross sectional view showing a fourth example of lightemitting element according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Substrate]

.—Ga₂O₃ substrate is conductive and, therefore, a vertical-type LED inelectrode configuration can be made. As a result, the entire lightemitting element of the invention forms a current flowing path and,thereby, the current density can be lowered and the life of the lightemitting element can be elongated.

FIG. 1 shows resistivity measurements of .—Ga₂O₃ substrate. Measuringthe resistivity of .—Ga₂O₃ substrate with n-type conductivity, as shownin FIG. 1, a resistivity of about 0.1 .cm is obtained at roomtemperature. Furthermore, the temperature dependency of resistivity issmall in the temperature range where the light emitting element will beexactly used. Therefore, the light emitting element using the .—Ga₂O₃substrate has an excellent stability.

Due to offering the vertical-type LED in electrode configuration, thestep of etching its n-layer to expose to form an n-electrode thereon isnot needed. Therefore, the number of manufacturing steps can bedecreased and the number of chips obtained per unit area of substratecan be increased. The manufacturing cost could be lowered.

In contrast, when a sapphire substrate is used, the electrodeconfiguration must be horizontal. In this case, after thin layers ofIII-V compound semiconductor such as GaN is grown, the step of etchingand masking the n-layer to expose to form an n-electrode thereon isadditionally needed to install the n-electrode. Comparing with this,when the electrode configuration is vertical as the case of GaAs systemlight emitting elements, the steps of etching and masking the n-layer isnot needed.

When a SiC substrate is used, the lattice mismatch between SiC and GaNis substantially large. In case of SiC, multiple phases of 3C, 4H, 6H,15R etc. exist and, therefore, it is difficult to obtain a substrate ina single phase. Also, due to the very high hardness, the processabilityis not good and it is difficult to get a smooth substrate. Observing itin atom scale, there exist a lot of steps with different phases on thesurface of substrate. When a thin layer is grown on such a SiCsubstrate, it must have multiple crystalline types and different defectdensities. Namely, in growing the thin layer on the SiC substrate, anumber of cores with different crystalline qualities are first grown onthe substrate and then the thin layer is grown such that the cores arecombined. Therefore, it is extremely difficult to improve thecrystalline quality of the thin layer The lattice mismatch between SiCand GaN is reported theoretically 3.4%, but it is, in fact, considerablygreater than that value due to the above reasons.

In comparison with SiC, .—Ga₂O₃ is of a single phase and has a smoothsurface in atom scale. Therefore, .—Ga₂O₃ does not have such asubstantially large lattice mismatch as observed in SiC. From theviewpoint of bandgap, SiC, e.g., 6H-SiC has a bandgap of 3.03 eV and isnot transparent in the wavelength range of shorter than about 427 nm.Considering that the entire light emission range of Ill-V systemcompound semiconductors is about 550 to 380 nm, the available wavelengthrange of SiC is only about two thirds of the entire range. In contrast,.—Ga₂O₃ is transparent in the range of longer than about 260 nm, whichcovers the entire light emission range of III-V system compoundsemiconductors, and is available particularly in ultraviolet region.

The substrate applicable to the invention, though it is basically of.—Ga₂O₃, may be of an oxide that includes Ga(gallium) as the majorcomponent and, as the minor component, at least one selected from thegroup of Cu, Ag, Zn, Cd, Al, In, Si, Ge and Sn. The minor componentfunctions to control the lattice constant or bandgap energy. Forexample, the substrate may be (Al_(X)In_(Y)Ga_((1-X-Y)))₂O₃, where0≦x≦1, 0≦y≦1, and 0≦x+y≦1.

[Thermal Expansion Coefficient]

From the viewpoint of thermal expansion, comparing GaN having a thermalexpansion coefficient of 5.6×10⁻⁶/K, .—Ga₂O₃ has that of 4.6×10⁻⁶/K,which is nearly equal to sapphire (4.5×10⁻⁶/K) and more advantageousthan 6H-SiC (3.5×10⁻⁶/K). Thus, to reduce the difference of thermalexpansion coefficients therebetween is a key factor in enhancing thequality of grown film.

[Bulk Single Crystal]

The most advantageous point of .—Ga₂O₃ is that it can give a bulk singlecrystal. In the light emission region of near-infrared to red obtainedfrom, typically, GaAs system material, a bulk single crystal is alwaysavailable and it allows a thin layer having an extremely small latticemismatch to the substrate to be grown on the conductive substrate.Therefore, it is easy to make a light emitting element with low cost andhigh light emission efficiency.

In contrast, for GaN system and ZnSe system materials expected to give ablue LED, it is, in fact, impossible to give a bulk single crystal. Inthe filed of these material systems, it has been a great deal tried tomake a bulk single crystal that is conductive and transparent in thelight emission region and that has a sufficiently small latticemismatch. However, even now, this problem is not solved. The .—Ga₂O₃substrate of the invention can perfectly solve the problem. Theinvention enables the manufacture of the bulk single crystal with adiameter of 2 inches by EFZ method or FZ method and, thereby, the blueto ultraviolet LEDs can be developed in the same way as GaAs system LED.

[Manufacture of Ga₂O₃ single crystal by EFG method]

FIG. 2 shows a crucible that is used to manufacture a Ga₂O₃ singlecrystal by EFG method. The crucible 6 is used being inserted to an EFGpulling vessel (not shown). The crucible 6 is of, e.g., iridium and isprovided with a slit die 8 having a slit 8 a through the capillaryphenomenon of which .—Ga₂O₃ melt 9 is lifted.

The growth of single crystal by EFG method is performed as follows..—Ga₂O₃ as raw material is entered a predetermined amount into thecrucible 6, being heated to melt, and, thereby, .—Ga₂O₃ melt 9 isobtained. .—Ga₂O₃ melt 9 is lifted above the slit die 8 provided in thecrucible 6 through the capillary phenomenon of the slit 8 a to contact aseed crystal 7. Being cooled, a grown crystal 10 having an arbitrarysectional form is obtained.

In detail, the crucible 6 of iridium has an inner diameter of 48.5 mm, athickness of 1.5 mm, and a height of 50 mm. Into the crucible 6, Ga₂O₃of 75 g as raw material is entered. Then, the slit die 8 which is 3 mmthick, 20 mm wide, 40 mm high and 0.5 mm slit interval is set in thecrucible 6. The crucible 6 is kept 1,760. in ordinary atmosphere ofnitrogen and at oxygen partial pressure of 5×10⁻² under 1 atm, the seedcrystal 7 of .—Ga₂O₃ is contacted to the .—Ga₂O₃ melt 9 being liftedthrough the capillary phenomenon of the slit 8 a. The growth speed ofsingle crystal is 1 mm/h.

The single crystal is grown on the slit die 8 while being defined by theshape of the slit die 8 and, therefore, the thermal gradient at thecrystal growing interface is considerably smaller than CZ method.Further, .—Ga₂O₃ melt 9 is supplied through the slit 8 a and the crystalgrowth speed is higher than the diffusion speed of .—Ga₂O₃ melt 9 in theslit 8 a. Therefore, the evaporation of components included in .—Ga₂O₃Melt 9, i.e., a variation in composition of .—Ga₂O₃ melt 9 can beeffectively suppressed. As a result, a high-quality single crystal canbe obtained. Also, to increase the size of single crystal can be easilyachieved by increasing the slit die 8 because the shape of grown crystal10 is defined by the shape of the slit die 8. Thus, EFG method offersthe increased size and high quality of Ga₂O₃ single crystal, which werehard to achieve by CZ method.

[Manufacture of Ga₂O₃ Single Crystal by FZ Method]

FIG. 3 shows an infrared heating single-crystal growing apparatus thatis used to manufacture a Ga₂O₃ single crystal by FZ(Floating Zone)method. The infrared heating single-crystal growing apparatus 100includes: a silica tube 102; a seed rotation section 103 that holds androtates a seed crystal 107 of .—Ga₂O₃ (hereinafter referred to as “seedcrystal 107”); a raw-material rotation section 104 that holds androtates a polycrystalline raw material 109 of .—Ga₂O₃ (hereinafterreferred to as “polycrystalline raw material 109”); a heater 105 thatheats the polycrystalline raw material 109 to melt it; and a controller106 that controls the seed rotation section 103, raw-material rotationsection 104 and the heater 105.

The seed rotation section 103 includes: a seed chack 133 that holds theseed crystal 107; a lower rotating shaft 132 that rotates the seed chack133; and a lower drive section that drives the lower rotating shaft 132to rotate clockwise and move upward and downward.

The raw-material rotation section 104 includes: a raw-material chack 143that holds the top end 109 a of polycrystalline raw material 109; anupper rotating shaft 142 that rotates the raw-material chack 143; and anupper drive section 141 that drives the upper rotating shaft 142 torotate counterclockwise and move upward and downward.

The heater 105 includes: a halogen lamp 151 that heats thepolycrystalline raw material 109 from the diameter direction to melt it;an elliptic mirror 152 that accommodates the halogen lamp 151 andconverges light radiated from the halogen lamp 151 to a predeterminedportion of the polycrystalline raw material 109; and a power source 153that supplies power to the halogen lamp 151.

The silica tube 102 accommodates the lower rotating shaft 132, the seedchack 133, the upper rotating shaft 142, the polycrystalline rawmaterial 109, a single crystal 108 of .—Ga₂O₃ and the seed crystal 107.The silica tube 102 is structured such that mixed gas of oxygen gas andnitrogen gas as inert gas can be supplied and sealed therein.

The growth of .—Ga₂O₃ single crystal is performed as follows. First ofall, the seed crystal 107 and polycrystalline raw material 109 areprepared. The seed crystal 107 is obtained, e.g., by cutting out .—Ga₂O₃along the cleaved surface. It has a diameter of less than one fifth ofthat of grown crystal or a sectional area of less than 5 mm² and hassuch a strength that it does not break when growing the .—Ga₂O₃ singlecrystal. The polycrystalline raw material 109 is obtained by charging apredetermined amount of Ga₂O₃ powder into a rubber tube(not shown),cold-compressing it at 500 MPa, then sintering it at 1500 for ten hours.

Then, the end of the seed crystal 107 is fixed to the seed chack 133,and the top end 109 a of polycrystalline raw material 109 is fixed tothe raw-material chack 143. The top end of the seed crystal 107 iscontacted to the bottom end of the polycrystalline raw material 109 bycontrolling upward and downward the position of the upper rotating shaft142. The positions of upper rotating shaft 142 and the lower rotatingshaft 132 are controlled upward and downward such that light of thehalogen lamp 151 is converged on the top end of the seed crystal 107 andthe bottom end of the polycrystalline raw material 109. The atmosphere102 a in the silica tube 102 is filled with mixed gas of nitrogen andoxygen, the composition of which may vary between 100% nitrogen and 100%oxygen, at a total pressure of 1 to 2 atm.

When an operator turns on a power switch(not shown), the controller 106starts the single crystal growth control to the respective sectionsaccording to a control program as follows. Turning on the heater 105,the halogen lamp 151 starts heating the top end of the seed crystal 107and the bottom end of the polycrystalline raw material 109, meltingtheir contacting portions to make a melt drop. In this stage, only theseed crystal 107 is kept rotating.

Then, the seed crystal 107 and the polycrystalline raw material 109 arerotated in the opposite direction to each other such that theircontacting portions are melted while being sufficiently mixed together.When a suitable amount of single crystal melt 108′ of .—Ga₂O₃ isobtained, the polycrystalline raw material 109 stops rotating and onlythe seed crystal 107 keeps rotating, and then the polycrystalline rawmaterial 109 and the seed crystal 107 are pulled in the oppositedirection to each other, i.e., upward and downward respectively to forma dash-neck which is thinner than the seed crystal 107.

Then, the seed crystal 107 and the polycrystalline raw material 109 areheated by the halogen lamp 151 while being rotating at 20 rpm in theopposite direction to each other, and the polycrystalline raw material109 is pulled upward at a rate of 5 mm/h by the upper rotating shaft142. In heating the polycrystalline raw material 109, thepolycrystalline raw material 109 is melt to form the single crystal melt108′, which is then cooled to grow .—Ga₂O₃ single crystal 108 that has adiameter of the same as or less than the polycrystalline raw material109. When a suitable length of single crystal is grown, the .—Ga₂O₃single crystal 108 is extracted.

Next, the manufacture of substrate using .—Ga₂O₃ single crystal 108 isconducted as follows. .—Ga₂O₃ single crystal 108 has a strong cleavageat (100) face when it is grown in b-axis <010> orientation and is,therefore, cleaved at planes parallel and vertical to (100) face to geta substrate. Alternatively, when it is grown in a-axis <100> orientationor c-axis <001> orientation, the cleavage at (100) or (001) face becomesweak and, therefore, the processability at all planes becomes good andthere is no limitation about the cleaved surface.

Advantages in the manufacture of .—Ga₂O₃ single crystal 108 by FZ methodare as follows.

(1) Large sized .—Ga₂O₃ single crystal 108 having a diameter of morethan 1 cm can be obtained since it is grown in a predetermineddirection.

(2) High crystalline quality can be obtained while suppressing thecracking and eutectic crystallization when .—Ga₂O₃ single crystal 108 isgrown in a-axis <100>, b-axis <010> or c-axis <001> orientation.

(3) .—Ga₂O₃ single crystal 108 can be produced in good reproducibilityand can be, therefore, applied to as a substrate for varioussemiconductor devices.

[Growth of II-VI Group Compound ZnSe Thin Film on .—Ga₂O₃ Substrate]

On (101) face of .—Ga₂O₃ system single crystal substrate, ZnSe thin filmwith p-type conductivity is grown at 350 by MOCVD (metal organicchemical vapor deposition). Source gases for ZnSe are Dimethyl zinc andH2Se. Nitrogen as p-dopant is supplied by preparing NH3 atmosphere.Nitrogen is, as acceptor, doped while being substituted for Se. II groupelement may be Zn, Cd and Hg, and IV group element may be O, S, Se, Teand Po. II-VI system compound applicable to the invention is, forexample, ZnSe, ZnO etc.

[Growth of III-V Group Compound Thin Film on .—Ga₂O₃ Substrate]

III-V group compound thin film is grown by MOCVD. III group element maybe B, Al, Ga, In and Tl, and V group element may be N, P, As, Sb and Bi.III-V system compound applicable to the invention is, for example, GaN,GaAs etc.

FIG. 4 shows an atomic arrangement in the case that GaN thin film isgrown on (101) face of .—Ga₂O₃ system single crystal substrate. In thiscase, (001) face of GaN is grown on (100) face of .—Ga₂O₃ system singlecrystal. O atoms 70, 70, . . . are arranged on (101) face of .—Ga₂O₃system single crystal. In FIG. 4, oxygen (O) atoms 70 are represented bysolid-line circle. The lattice constant of .—Ga₂O₃ system single crystalin (101) face is a=b=0.289 nm, .□116°. The lattice constant of GaN in(001) face is a_(G)=b_(G)=0.319 nm, .G=120°. In FIG. 4, nitrogen (N)atoms 80 are represented by dot-line circle.

When GaN thin film is formed such that (001) face of GaN is grown on(101) face of .—Ga₂O₃ system single crystal, the mismatch of latticeconstant is about 10%, and the mismatch of angle is about 3%. Thus, thearrangement of oxygen atoms of Ga₂O₃ system single crystal is nearlyidentical to that of nitrogen atoms of GaN, and, therefore, GaN thinfilm can have a uniform plane structure. Even when GaN thin film isformed on (101) face of .—Ga₂O₃ system single crystal without a bufferlayer to be inserted therebetween, there is no problem about latticemismatch.

Alternatively, when In is added to .—Ga₂O₃ single crystal to adjust thelattice constant, the lattice constant of GaN in (001) face becomescloser to that of .—Ga₂O₃ system single crystal in (101) face.Therefore, GaN thin film can have a more uniform plane structure.

FIG. 5 shows an atomic arrangement in the case that GaN thin film isgrown on Al₂O₃ system crystal substrate. O atoms 75, 75, . . . arearranged on (001) face of Al₂O₃ system crystal. In FIG. 5, oxygen (O)atoms 75 are represented by solid-line circle. The lattice constant ofAl₂O₃ system crystal in (001) face is a_(A)=b_(A)=0.475 nm, ._(A)=120°.The lattice constant of GaN in (001) face is a_(G)=b_(G)=0.319 nm,._(G)=120°. In FIG. 5, nitrogen (N) atoms 80 are represented by dot-linecircle.

When GaN thin film is formed such that (001) face of GaN is grown on(001) face of Al₂O₃ system crystal, the mismatch of lattice constant isabout 30%. Therefore, when GaN thin film is formed on Al₂O₃ systemcrystal without a buffer layer to be inserted therebetween, there occursa problem that due to the lattice mismatch therebetween GaN thin filmmay have no uniform plane structure.

[Method of Growing Thin Film on .—Ga₂O₃ Substrate]

FIG. 6 schematically shows a MOCVD apparatus. The MOCVD apparatus 20includes: a reaction vessel 21 equipped with an exhaust system 26 thatincludes a vacuum pump and an exhauster (not shown); a susceptor 22 onwhich a substrate 27 is mounted; a heater 23 to heat the susceptor 22; acontrol shaft that rotates and moves upward and downward the susceptor22; a silica nozzle 25 that supplies source gases in the oblique orhorizontal direction to the substrate; a TMG gas generator 31 thatgenerates trimethyl gallium (TMG) gas; a TMA gas generator 32 thatgenerates trimethyl aluminum (TMA) gas; and a TMI gas generator 33 thatgenerates trimethyl indium (TMI) gas. If necessary, the number of gasgenerators may be increased or decreased. Nitrogen source is NH₃ andcarrier gas is H₂. When growing GaN thin film, TMG and NH₃ are used.When growing AlGaN thin film, TVA, TMG and NH₃ are used. When growingInGaN thin film, TMI, TMG and NH₃ are used.

FIG. 7 shows the cross sectional structure of a light emitting element40 including semiconductor layers grown by the MOCVD apparatus 20 inFIG. 6.

The process of growing the semiconductor layers by the MOCVD apparatus20 is as follows. First of all, the substrate 27 is mounted on thesusceptor 22 while facing up the film-forming surface, and then it isset into the reaction vessel. At a temperature of 1020, TMG of 54×10⁻⁶mol/min, NH₃ of 4 l/min, H₂ of 2 l/min and monosilane (SiH₄) of 2×10⁻¹¹mol/min are flown for 60 min, thereby 3 . . . thick Si-doped GaN (n-GaN)layer la is grown.

Then, at a temperature of 1030, TMG of 54×10⁻⁶ mol/min, NH₃ of 4 l/min,H₂ of 2 l/min and biscyclopentadienyl magnesium (CP₂Mg) of 3.6×10⁻⁶mol/min are flown for 20 min, thereby 1 . . . thick Mg-doped GaN (p-GaN)layer 1 b is grown. On the layer 1 b, transparent electrode (Au/Ni) 1 his deposited and then Mg-doped GaN 1 b is made p-type by annealing. Abonding electrode (p-electrode) 1 c is formed on the transparentelectrode 1 h, and a bonding wire 1 f is bonded to the bonding electrode1 c while forming a ball 1 e. Then, an n-electrode 1 d is formed on thebottom surface of the substrate 27. Thus, the light emitting element 40is obtained.

The electrodes are formed by deposition or sputtering etc. They are ofmaterials that offer ohmic contact to the layer or substrate on whichthe electrodes are formed. For example, to n-type conductivity layer orsubstrate, any one of metals including Au, Al, Co, Ge, Ti, Sn, In, Ni,Pt, W, Mo, Cr, Cu, and Pb, or an alloy including two or more of themetals (e.g., Au—Ge alloy), or two-layer structure selected from themetals (e.g., Al/Ti, Au/Ni and Au/Co), or ITO (indium tin oxide) isused. To p-type conductivity layer or substrate, any one of metalsincluding Au, Al, Be, Ni, Pt, In, Sn, Cr, Ti and Zn, or an alloyincluding two or more of the metals (e.g., Au—Zn alloy and Au—Be alloy),or two-layer structure selected from the metals (e.g., Ni/Au), or ITO isused.

[Forming of Thin Layers with Different Carrier Concentrations]

For example, on n-GaN layer, another n-GaN layer with a carrierconcentration lower than the n-GaN layer is formed, and, on the lowercarrier concentration n-GaN layer, p-GaN layer and another p-GaN layerwith a carrier concentration higher than the p-GaN layer are formed inthat order. The carrier concentration can be differentiated by changingthe amount of n-dopant or p-dopant added.

When using the substrate of .—Ga₂O₃ system single crystal and forming,on the substrate, a plurality of n-type layers with different carrierconcentrations and a plurality of p-type layers with different carrierconcentrations, advantages (1) to (4) stated below are obtained.

(1) Due to a carrier concentration lower than the substrate, the n-GaNlayer grown on the substrate has a good crystalline quality and therebythe light emission efficiency is enhanced.

(2) Due to the junction of n-GaN layer and p-GaN layer, the lightemitting element having pn-junction is formed and therefore the lightemission of a short wavelength is obtained through the bandgap of GaN.

(3) Due to being of .—Ga₂O₃ system single crystal, the substrate canenjoy a high crystalline quality and n-type good conductivity.

(4) Due to being of .—Ga₂O₃ system single crystal, the substrate cantransmit light of ultraviolet region and therefore ultraviolet light tovisible light can be emitted from the substrate side.

[Forming of Buffer Layer]

FIG. 8 shows a modification that the light emitting element in FIG. 7further includes a buffer layer. Between .—Ga₂O₃ single crystalsubstrate 27 of the invention and n-GaN layer 1 a, there is formed aAl_(X)Ga_(1-X)N buffer layer (0≦x≦1). The buffer layer is grown by theabove-mentioned MOCVD apparatus. The p-n junction structure is grown bythe above-mentioned method of growing thin film on .—Ga₂O₃ substrate.

Examples of this invention are stated below.

EXAMPLE 1 Forming of n-GaN Thin Film on P-Type Conductivity Substrate

The p-type conductivity substrate is made as follows. First, .—Ga₂O₃single crystal is prepared by FZ method. The .—Ga₂O₃ polycrystalline rawmaterial is obtained by uniformly mixing, for example, .—Ga₂O₃ includingMgO (p-dopant source) and charging a predetermined amount of the mixtureinto a rubber tube, cold-compressing it at 500 MPa to form a stick, thensintering it at 1500. for ten hours in the atmosphere. Thereby, .—Ga₂O₃system polycrystalline raw material including Mg is obtained. By anotherway, .—Ga₂O₃ seed crystal is provided. Under the growth atmosphere withtotal pressure of 1 to 2 atm, flowing mixture gas of N₂ and O₂ at 500ml/min, the .—Ga₂O₃ seed crystal and .—Ga₂O₃ system polycrystalline rawmaterial are contacted to each other in the silica tube, and they areheated such that the .—Ga₂O₃ seed crystal and .—Ga₂O₃ systempolycrystalline raw material are melted at the contacting portion. Themelting .—Ga₂O₃ seed crystal and .—Ga₂O₃ system polycrystalline rawmaterial are rotated at 20 rpm in the opposite direction to each other,and .—Ga₂O₃ single crystal is grown at a rate of 5 mm/h during therotation. As a result, on the .—Ga₂O₃ seed crystal, transparent andinsulating .—Ga₂O₃ system single crystal including Mg is obtained. TheGa₂O₃ system single crystal is used as the substrate. The substrate isthen annealed at a predetermined temperature (e.g., 950) in oxygenatmosphere for a period and, thereby, the number of oxygen defects isdecreased to give the p-type conductivity substrate.

Then, the n-type conductivity thin film is formed on the substrateobtained. The thin film is grown by MOCVD method. First, the p-typeconductivity substrate is set into the MOCVD apparatus. Keeping thesubstrate temperature at 1150., H₂ of 20 l/min, NH₃ of 10 l/min, TMG of1.7×10⁻⁴ mol/min and monosilane (SiH₄) diluted to 0.86 ppm by H₂ of 200m/min are flown for 30 min. Thereby, about 2.2 . . . thick n-typeconductivity GaN thin film with a carrier concentration of 1.5×10¹⁸/cm³is formed.

EXAMPLE 2 Light Emitting Element with pn-Junction

FIG. 9 shows a light emitting element with pn-junction mounted on aprinted circuit board. The light emitting element 40 includes: a Ga₂O₃substrate 41 of .—Ga₂O₃ single crystal; a Al_(X)Ga_(1-X)N buffer layer42 (0≦x≦1) formed on the Ga₂O₃ substrate 41; a n-GaN layer 43 formed onthe Al_(X)Ga_(1-X)N buffer layer 42; a p-GaN layer 44 formed on then-GaN layer 43; a transparent electrode 45 formed on the p-GaN layer 44;a Au bonding electrode 47 formed on part of the transparent electrode45; and a n-electrode 46 formed on the bottom surface of the Ga₂O₃substrate 41. The light emitting element 40 is mounted on the printedcircuit board 50 through a metal paste 51 and a bonding wire 49 isbonded to the bonding electrode 47 while forming a ball 48.

The light emitting element 40 emits light at the pn-junction interfacewhere the n-GaN layer 43 and p-GaN layer 44 are bonded. Emitted light isoutput such that part of emitted light is output, as output light 60,upward through the transparent electrode 45 and another part is firstdirected to the bottom of the Ga₂O₃ substrate 41, transmitting throughthe substrate 41, then being outputted upward after being reflected bythe metal paste 51. Thus, the light emission intensity of is increasedcomparing the case that emitted light is directly output.

EXAMPLE 3 Flip-Chip Type Light Emitting Element

FIG. 10 shows a flip-chip type light emitting element. The lightemitting element 40 includes: a Ga₂O₃ substrate 41 of .—Ga₂O₃ singlecrystal; a Al_(X)Ga_(1-X)N buffer layer 42 (0≦x≦1) formed on the Ga₂O₃substrate 41; a n-GaN layer 43 formed on the Al_(X)Ga_(1-X)N bufferlayer 42; a p-GaN layer 44 formed on part of the n-GaN layer 43; an-electrode 46 formed on the n-GaN layer 43; and a p-electrode 52 formedon the p-GaN layer 44. The light emitting element 40 is flip-chip bondedthrough solder balls 63, 64 beneath the p-electrode 52 and n-electrode46 to lead frames 65, 66.

The light emitting element 40 emits light at the pn-junction interfacewhere the n-GaN layer 43 and p-GaN layer 44 are bonded. Emitted light isoutput, as output light 60, upward transmitting through the Ga₂O₃substrate 41.

EXAMPLE 4 Double-Heterostructure Light Emitting Element

FIG. 11 shows a double-heterostructure light emitting element. The lightemitting element 40 includes: a Ga₂O₃ substrate 41 of .—Ga₂O₃ singlecrystal; a Al_(Y)Ga_(1-Y)N buffer layer 42 (0≦y≦1) formed on the Ga₂O₃substrate 41; a n-Al_(Z)Ga_(1-Z)N cladding layer 55 (0≦z.1) formed onthe Al_(Y)Ga_(1-Y)N buffer layer 42; a In_(m)Ga_(1-m)N light-emittinglayer 56(0≦m.1) formed on the n-Al_(Z)Ga_(1-Z)N cladding layer 55; ap-Al_(p)Ga_(1-p)N cladding layer 57 (0≦p.1,p.Z) formed on theIn_(m)Ga_(1-m)N light-emitting layer 56; a transparent electrode 45formed on the p-Al_(P)Ga_(1-P)N cladding layer 57; a Au bondingelectrode 47 formed on part of the transparent electrode 45; and an-electrode 46 formed on the bottom surface of the Ga₂O₃ substrate 41.The light emitting element 40 is mounted on the printed circuit board 50through a metal paste 51 and a bonding wire 49 is bonded to the bondingelectrode 47 while forming a ball 48.

The bandgap energy of the n-Al_(Z)Ga_(1-Z)N cladding layer 55 is greaterthan that of the In_(m)Ga_(1-m)N light-emitting layer 56, and thebandgap energy of the p-Al_(P)Ga_(1-P)N cladding layer 57 is greaterthan that of the In_(m)Ga_(1-m)N light-emitting layer 56.

The light emitting element 40 has the double-heterostructure whereelectron and hole as carriers are confined in the In_(m)Ga_(1-m)Nlight-emitting layer 56 to increase the probability of recombinationtherebetween. Therefore, the light emission efficiency can be remarkablyenhanced.

Furthermore, emitted light is output such that part of emitted light isoutput, as output light 60, upward through the transparent electrode 45and another part is first directed to the bottom of the Ga₂O₃ substrate41, transmitting through the substrate 41, then being outputted upwardafter being reflected by the metal paste 51. Thus, the light emissionintensity of is increased comparing the case that emitted light isdirectly output.

ADVANTAGES OF THE INVENTION

(1) According to the invention, a light emitting element and a method ofmaking the same that use the transparent and conductive substrate ofbulk .—Ga₂O₃ single crystal can be provided. The light emitting elementcan be equipped with electrodes formed on the top and bottom and,therefore, the structure can be simplified to enhance the manufacturingefficiency. Also, the efficiency of light outputted can be enhanced.(2) Due to employing the .—Ga₂O₃ system material, the substrate can becolorless, transparent and conductive. It can transmit visible light toultraviolet light. The light emitting element made by using thesubstrate can have the vertical structure. Also, emitted light can beoutputted from the substrate side.(3) Furthermore, the .—Ga₂O₃ single crystal substrate has aprocessability better than a substrate of conventional materials, i.e.,sapphire and SiC.Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A light emitting element, comprising: a Ga₂O₃ substrate; an n-typeAl_(z)Ga_(1-z)N cladding layer formed on a first surface of the Ga₂O₃substrate, where 0≦z<1; an In_(m)Ga_(1-m)N light emitting layer formedon the n-type AlGaN cladding layer, where 0≦m<1; a p-typeAl_(p)Ga_(1-p)N cladding layer formed on the InGaN light emitting layer,where 0≦p<1 and p>z; an n-side electrode provided on a second surface ofthe Ga₂O₃ substrate; and a p-side electrode provided on the p-typeAl_(p)Ga_(1-p)N cladding layer.
 2. A light emitting clement according toclaim 1, further comprising: a Al_(y)Ga_(1-y)N buffer layer formedbetween the Ga₂O₃ substrate and the n-type Al_(z)Ga_(1-z)N claddinglayer, where 0≦y<1.
 3. The light emitting element, according to claim 2,wherein the p-side electrode includes a transparent electrode providedon the p-type Al_(p)Ga_(1-p)N cladding layer, and a bonding electrodeprovided on the transparent electrode.
 4. The light emitting elementaccording to claim 1, further comprising: a printed circuit board onwhich the light emitting element is mounted through a metal paste whichreflects emitted light transmitted through the Ga₂O₃ substrate.